The present invention relates to hot-worked permanent magnets consisting substantially of rare earth elements, transition metals and boron and provided with magnetic anisotropy by hot working, and more particularly to hot-worked magnets having improved crystal grain orientation and thus having good magnetic properties. It also relates to a method of producing such hot-worked magnets without cracking by adding proper amounts of additives to improve their workability.
Permanent magnets consisting essentially of rare earth elements, transition metals and boron (hereinafter referred to as "R-T-B permanent magnets") have been getting much attention as inexpensive permanent magnets having excellent magnetic properties. This is because intermetallic compounds expressed by R.sub.2 T.sub.14 B having a tetragonal crystal structure have excellent magnetic properties. Nd.sub.2 Fe.sub.14 B, in which Nd is employed as R, has lattice parameters of a.sub.0 =0.878 nm and C.sub.0 =1.218 nm.
The R-T-B permanent magnets are usually classified into two groups: sintered magnets and rapidly quenched magnets. Whichever production method is utilized, it is necessary to form them to desired shapes. In this sense, they should have good workability. In order to improve the workability of the magnets, the addition of lubricating agents has conventionally been conducted. The lubricants are classified into external lubricants which are applied to die surfaces or surfaces of magnet products to be formed to reduce a friction coefficient between the die surfaces and the magnet products being formed, and internal lubricants which are in the form of powder, liquid, solid, etc. and added to the magnet products to be formed to reduce a friction coefficient between powder particles.
In the case of sintered magnets, stearic acid is widely used as an internal lubricant (Japanese Patent Laid-Open No. 61-34101). Here, stearic acid is a saturated aliphatic acid having the formula: CH.sub.3 (CH.sub.2).sub.16 COOH.
Incidentally, it is known to suppress the growth of crystal grains and simultaneously increase the density of the resulting magnet in the sintering step by adding carbon powder or powder of carbide-forming components such as Ti, Zr, Hf, etc. to form metal carbides (Japanese Patent Laid-Open No. 63-98105).
However, if sintered magnets are to be provided with magnetic anisotropy, a pressing step in a magnetic field would have to be conducted, limiting the shapes of magnets to be formed.
In view of this fact, much attention has come to be paid to rapidly quenched magnets which do not need the pressing in a magnetic field, particularly permanent magnets obtained by pulverizing thin ribbons or flakes produced from melts of R-T-B alloys by a rapid quenching method, hot-pressing them (high-temperature treatment) and then subjecting them to plastic working at high temperature to provide them with magnetic anisotropy, which will be called "hot-worked magnets" hereinafter) (European Patent Laid-Open No. EP 0,133,758). The thin ribbons or flakes produced by a rapid quenching method usually contain innumerable fine crystal grains. Even though the thin ribbons or flakes produced by a rapid quenching method are in various planar shapes of 30 .mu.m in thickness and 500 .mu.m or less in length, the crystal grains contained therein are as fine as 0.02-1.0 .mu.m in an average grain size, which is smaller than the average grain size of 1-90 .mu.m in the case of sintered magnets (for instance, European Patent Laid-Open No. EP 0,126,179). The average grain size of the rapidly quenched magnets is close to 0.3 .mu.m, the critical size of a single domain of the R-T-B magnet, which means that it provides essentially excellent magnetic properties.
In the case of hot working of the rapidly quenched magnetic materials, it is important that there is a close relationship between the direction of their plastic flow and their magnetic orientation perpendicular to the direction of the plastic flow. Further, it is necessary to cause the plastic flow uniformly in the entire magnet to be worked, in order to improve the orientation of the crystal grains having close relations with magnetic properties. Incidentally, a nonuniform deformation may cause bulging of the magnets in the plastic working process, which in turn produces large or many cracks in the peripheral portions of the magnets. This is a serious problem when hot-worked magnets are to be obtained in the shape of final products.
Most of force applied in a hot-working process is used for plastic deformation, but part of the force is exhausted by friction. This may be partially the cause of the above bulging phenomenon.
European Patent Laid-Open No. EP 0,133,758 discloses the coating of a die surface with graphite as an external lubricant for hot die-upsetting, to improve the workability of magnets in the hot-working process, thereby obtaining hot-worked magnets free from cracks. Incidentally, the effects of graphite on the inner lubrication of the magnets are not referred to.
In the above-mentioned conventional techniques, graphite applied to the die surface for die lubrication is only partly, if any, attached to thin ribbons or flakes produced by a rapid quenching method, which are 30 .mu.m or so in thickness and 500 .mu.m or less in length, much less to innumerable fine crystal grains inside the thin flakes.
Incidentally, in the case of adding carbon powder or powder of carbide-forming components such as Ti, Zr, Hf, etc. to sintered magnets, it is expected that such powder is relatively easily dispersed in magnet powder by appropriately selecting a powder shape and a mixing method. The same is true of stearate. This is because in the case of sintered magnets, magnetic powder particles produced by pulverizing alloy ingots are in a shape close to sphere.
However, unlike the sintered magnets produced by a powder metallurgy method in which compacting is conducted at room temperature, in the case of hot-working such as die-upsetting, it is usually conducted at as high a temperature as 600-850.degree. C. Accordingly, additives dispersed among thin flakes show essentially different functions, and this has not yet been paid any attention so far.
In addition, in the conventional techniques in which an external lubricant is applied to a die surface, they do not show effects peculiar to the hot working of the magnets, but they simply show effects of lubricants which slightly decrease a friction coefficient between the die surface and materials being worked. In fact, there has been no report so far with respect to the improvement of workability without remarkable cracking and the improvement of uniform orientation in the field of hot-working of rapidly quenched magnet ribbons or flakes.